This invention relates to flexible hose assemblies, and in particular to a new and improved flexible hose assembly for a mercury line for a mercury manometer.
Precision mercury manometers are shown in U.S. Pat. Nos. 3,225,599 and 3,296,867. These instruments utilized a flexible line several feet in length which line is filled with mercury. The instrument is adversely affected by leakage of mercury, and therefore the flexible line and its fittings should be leakproof. In a conventional instrument, the line and its fittings are required to pass a leak test at 5.times.10.sup.-9. While it is relatively easy to obtain flexible hose which will meet this requirement over the operating life of the instrument, problems are encountered in the joining of the hose to the hose fittings by which the hose is interconnected with the components of the instrument.
The leakage test requirement referred to herein is a leak rate of less than 5.times.10.sup.-9 standard cubic centimeters of test gas per second, which typically is measured with a Hastings calibrated gas leak instrument. This rate is essentially zero leakage for most situations.
In a typical instrument, the flexible hose is formed of a convoluted metal covered by a protective woven metal braid, with a fitting welded to the hose at each end. All mercury line components usually are stainless steel. In one prior art arrangement, a helical hose with a covering braid is provided with a sleeve at the hose end, with the sleeve, braid and hose being welded to the end of the fitting. Obtaining a satisfactory weld which will pass the leak test and also operate satisfactorily during the life of the instrument has been very difficult. Unless the amount of heat is very carefully controlled during the welding operation, portions of the hose adjacent the weld can be damaged and the assembly will not meet the leakage requirement.
In another prior art item utilizing a circular convoluted hose, the fitting has an end and also a welding section spaced from the end. The fitting end is welded to the hose end. Then the braid over the hose and a sleeve over the braid are welded to the welding section of the fitting in a separate operation. This arrangement has been satisfactory with circulated convoluted hose but has not worked well with helical convoluted hose.